As far as the bed liner, are you planning on taking to entire liner out, or just the area to be welded ?
We completely removed a green colored liner out of my sons 02 silverado that was at least a 1/2" thick (looked like two liners on top of each other done in someone garage
). We found that if you get a razor knife and cut it in 1-1/2" - 2" strips and then use a air chisel / hammer with a flat blade on it, the liner will come right out. Its was a long process, but we got it done. Had a new black liner sprayed in and it looks a whole lot better !